I. Field of the Invention
The present invention relates to an improvement of a sealed metal oxide-hydrogen cell comprising an anode formed of a hydrogen storage alloy.
II. Description of the Prior Art
Recently, a sealed metal oxide-hydrogen cell comprising a cathode formed of a metal oxide and an anode formed of a hydrogen storage alloy attracts attention as a cell exhibiting a high energy density. The basic construction of the cell, which belongs to the alkaline cell, should be equal to that of the nickel-cadmium cell. What is particularly important in the cell of this type is the capacity balance between the cathode and the anode.
Generally, the anode capacity is greater than the cathode capacity in the sealed alkaline cell. In the product cell after completion of all the manufacturing steps, it is necessary for the excess portion of the anode capacity to be partly charged. In this case, the residual excess portion of the anode capacity should remain uncharged even under the condition that the cell has been charged for the cathode to reach the full charge state. In other words, the anode does not reach the full charge state even if the full charge state has been reached in the cathode, with the result that the hydrogen generation from the anode can be suppressed. It follows that it is possible to prevent the internal pressure of the cell from being elevated by the hydrogen generation even under the overcharge. Also, the excess portion of the anode capacity is partly charged in advance as described above in order to enable the cathode to fully exhibit its performance and to prevent the charge from being completely released even under a slight overdischarge. As a result, the cell is prevented from being deteriorated so as to ensure a long life of the cell.
In assembling a cell having the capacity balance of the cathode and anode described above, it is necessary to charge in advance the anode in the required amount corresponding to a portion of the excess capacity of the anode in question. Thus, in the nickel-cadmium cell, the cadmium anode is generally charged in the formation step. The cadmium anode charged as required in the formation step is incorporated after water-wash and drying into a sealed container together with the cathode.
However, a serious problem is brought about, if the pre-charge process of the cadmium anode described above is applied to the hydrogen storage alloy anode of the sealed metal oxide-hydrogen cell. Specifically, since the charged active material is hydrogen, hydrogen is readily released from the hydrogen storage alloy anode in the process after the formation step. It should be noted that, under the air atmosphere, the released hydrogen is burned on the surface of the hydrogen storage alloy anode. Under the inert gas atmosphere, burning does not take place but hydrogen is promptly released to the outer atmosphere.
To prevent the hydrogen release problem, it is necessary to carry out all the steps after the formation step under the atmosphere having a hydrogen partial pressure higher than the equilibrium plateau pressure of the hydrogen storage alloy which forms the anode. In this case, however, some steps of the cell assembling process are carried out under the hydrogen atmosphere, with the other steps being carried out under the air atmosphere. It follows that air tends to enter the hydrogen atmosphere, and vice versa, in the cell assembling process. Naturally, the mixing of the air with hydrogen possibly causes ignition of hydrogen. In order to prevent the hydrogen ignition problem, a large facility and troublesome operations are required. In short, serious problems must be solved for pre-charging a part of the capacity of the anode of a hydrogen storage alloy in the formation step.